The present disclosure is directed to electro-permanent magnetic devices, and more particular to methods of controlling power to electro-permanent magnetic devices and related controllers.
Electro-permanent magnetic devices are used in industrial systems, for example, to magnetically clamp objects securely without fear of loss of magnetic clamping force due to power failures. Applications include holding workpieces during machining, holding dies/molds in presses, lifting, transporting, etc.
The magnetic circuit of an electro-permanent magnetic device includes an electro-permanent magnet that can be electrically switched between On (magnetized) and Off (de-magnetized) states, and once in either state, no further electrical energy is required to maintain the electro-permanent magnet in that state. Once energized, the electro-permanent magnet can thus be disconnected from electrical power indefinitely without losing magnetic clamping power.
The ability to disconnect an electro-permanent magnetic device from power without losing magnetic clamping power is valuable because it allows a part to be loaded on an electro-permanent magnetic chuck, and then the chuck can be disconnected from power and moved through multiple processing stations without the need of an electrical connection being maintained with the chuck and without fear of losing holding power.
FIGS. 1A-B and 2A-B illustrate examples of circuits of electro-permanent magnetic devices. The DM (double magnet) circuit of FIGS. 1A and 1B uses two types of magnets, a switching magnet including a switching magnetic material (having relatively low coercivity) 101 and a coil 103 (used to switch a magnetic state of the switching magnetic material 101 responsive to an electrical current through the coil 103) and a non-switching magnet including a non-switching magnetic material (having relatively high coercivity) 105. The DM circuit is switched by changing the state (i.e., polarity) of the switching magnetic material 101 such that magnetic energy is either trapped inside the unit in a de-magnetized or off state as shown in FIG. 1A or pushed out through the poles 107 to clamp a component in a magnetized or on state as shown in FIG. 1B. While not shown in cross section in FIGS. 1A and 1B, an electro-permanent magnetic device may include a two-dimensional array of switching magnets (including switching magnetic material 101 and coil 103), non-switching magnets (including non-switching magnetic material 105), and pole pieces 107. Moreover, the coils 103 of the switching magnets may be electrically coupled in series or in parallel, with a controller providing electrical current through the coils 103 to switch the switching magnets on and off. In the DM circuit of FIGS. 1A and 1B, the switching magnets are switched between on in a first polarity to offset the magnetic force of non-switching magnets 105 (e.g., as shown in the de-magnetized off state of FIG. 1A) and on in a second polarity (opposite the first polarity) to add to the magnetic force of the non-switching magnets 105 (e.g., as shown in the magnetized on state of FIG. 1B).
The SM (single magnet) circuit of FIGS. 2A and 2B omits the non-switchable magnets and switches the switchable magnets 201 on (magnetized) and off (de-magnetized) in a manner similar to that discussed above with respect to FIGS. 1A and 1B. In the SM circuit of FIGS. 2A and 2B, however, the switchable magnets are switched between off (demagnetized) and on (magnetized). The SM circuit of FIGS. 2A and 2B switches off by actually using the coil 203 to de-magnetize the switching magnetic material 201. When switched off, the switching magnets 201 may be substantially demagnetized as shown in FIG. 2A, and when switched on, magnetic energy from the switching magnets 201 may be pushed out through the poles 207 as shown in FIG. 2B.
Devices using switching magnets are discussed, by way of example, in U.S. Pat. No. 7,999,645, in U.S. Pat. No. 6,292,078, in U.S. Pat. No. 4,847,582, and in U.S. Pat. No. 4,507,635, the disclosures of which are hereby incorporated herein in their entireties by reference.
Both DM circuits and SM circuits can use a same or similar control technology, with a difference being that the Off cycle for the SM circuit may be longer and/or more complex involving a diminishing +/− sequence of firing pulses to demagnetize the switching magnetic material.
Two basic options to control these magnetic systems include DC rectified control and AC chopped wave “impulse magnetizer” control. AC impulse magnetizer control is most commonly used in industry. Very large systems may be difficult to manage using AC impulse magnetizer control, however, and DC rectified control techniques may be used instead. Currently, many applications may use AC impulse magnetizer control techniques.
In both DM and SM circuits, relatively high voltages and/or currents may thus be used to switch the switching magnets to provide the magnetized (on) and de-magnetized (off) states of electro-permanent magnetic devices. The use of such high voltages/currents in harsh operating environments (e.g., including the presence of metal shavings, cooling liquids, etc.) may increase a risk of an electrical short circuit. Accordingly, control systems that reduce risk of damage and/or injury resulting from electrical shorts (e.g., to ground, between phases, etc.) and/or uncontrolled current flow during magnetic switching operations may be useful.